1. Field of the Invention
The present invention relates to an inertization device and method for lowering the risk of fire in an enclosed protected area, in which the oxygen content in the protected area is maintained in a defined control range for a defined period at a control concentration that lies below an operating concentration, by feeding an oxygen-displacing gas from a primary source.
2. Description of the Related Art
Inertization methods for preventing and extinguishing fire in enclosed areas are known from fire extinguishing technology. The extinguishing effect resulting with this method is based on the principle of oxygen displacement. It is known that regular ambient air consists of 21% by volume oxygen, 78% by volume nitrogen and 1% by volume other gases. For extinguishing purposes, the nitrogen concentration in the affected area is increased further, for example by feeding pure nitrogen as an inert gas, thus lowering the oxygen percentage. It is a known fact that an extinguishing effect is achieved when the oxygen percentage drops to less than about 15% by volume. Depending on the combustible materials present in the affected area, further lowering of the oxygen percentage, for example to 12% by volume, may be required. At this oxygen concentration, most combustible materials are no longer able to burn.
The oxygen-displacing gases used with this “inert gas extinguishing technique” are generally stored in the compressed state in steel cylinders in special ancillary rooms. It is furthermore conceivable to use a device for producing a gas that will displace the oxygen. These steel cylinders and/or this device for producing the gas that will displace the oxygen constitute the primary source of the inert gas fire extinguishing system. Where necessary, the gas can then be conducted from this primary source via pipe systems and corresponding discharge nozzles into the affected area.
The associated inert gas fire extinguishing system generally includes at least one installation for the sudden feeding of oxygen-displacing gas from the primary source to the monitored area and a fire detection device for detecting a fire parameter in the air.
Designing the entire fire prevention and/or inert gas fire extinguishing system at the highest possible safety level necessitates equipment and logistics planning in the event of a system shutdown as a result of malfunctions in order to comply with safety requirements. While during the project engineering phase of the fire prevention and/or inert gas fire extinguishing system, all measures allowing the system to be restarted as quickly and smoothly as possible have been taken into consideration, the inertization by means of the inert gas technique is also associated with certain problems and has clear limits in terms of a fail-safe performance. It has turned out that while it is possible to design the inert gas fire extinguishing system such that the probability of the event of a malfunction during the lowering and/or control phases of the oxygen content in the protected area to a control concentration that is below a predefined operating concentration is relatively low, the problem often arises that the control concentration has to be maintained for an extended period of time, during the so-called emergency operation phase, at the required level, particularly because the inertization methods known from the prior art offer no possibility of preventing a re-ignition level of the oxygen concentration in the protected area from being exceeded too early if due to a malfunction the primary source fails completely or at least partially.
The re-ignition phase designates the time period following the fire fighting phase, during which the oxygen concentration in the protected area must not exceed a defined level—the so-called re-ignition prevention level—so as to prevent renewed ignition of the materials present in the protected area. The re-ignition prevention level is an oxygen concentration that depends on the fire load of the protected area and is determined on the basis of experiments. According to German VdS Guidelines, when flooding the protected area, the oxygen concentration in the protected area must reach the re-ignition prevention level of for example 13.8% by volume within the first 60 seconds following the start of flooding (fire fighting phase). Moreover, the re-ignition prevention level must not be exceeded within 10 minutes following the end of the fire fighting phase. To this end it is provided that the fire is completely extinguished in the protected area during the fire fighting phase.
With the inertization methods known from the prior art, the oxygen concentration is lowered as quickly as possible to a so-called operating concentration when a detection signal is issued. The required inert gas is provided by the primary source of the inert gas fire extinguishing system. The term “operating concentration” should be interpreted as a level below a so-called design concentration. The design concentration is an oxygen concentration in the protected area at which the combustion of any material present in the protected area is effectively prevented. When defining the design concentration of a protected area, for safety purposes generally a margin is deducted from the limit at which the combustion of any materials in the protected area is prevented. Upon reaching the operating concentration in the protected area, the oxygen concentration is typically maintained at a control concentration that is below the operating concentration.
The control concentration is a control range for the residual oxygen concentration in the inertized protected area, within which the oxygen concentration is maintained during the re-ignition phase. The control range is defined by an upper limit, the on-threshold for the primary source of the inert gas fire extinguishing system, and a lower limit, the off-threshold for the primary source of the inert gas fire extinguishing system. During the re-ignition phase, the control concentration is maintained in this control range by repeatedly feeding inert gas. The inert gas is typically provided from the reservoir of the inert gas fire extinguishing system that serves as the primary source, i.e., either the device for producing the oxygen-displacing gas (such as a nitrogen generator), gas bottles or other buffer devices. In the event of a malfunction or failure, the risk exists that the oxygen concentration in the protected area will increase prematurely and that the re-ignition prevention level will be exceeded, thus shortening the re-ignition phase and eliminating the guarantee that the fire in the protected area can be fought successfully.
Accordingly, an inert gas fire extinguishing system and/or an inertization method, that overcome these obstacles, is needed.